Cambrian organisms lived during the Cambrian era, from approximately 542 to 488 million years ago. The Cambrian era is famous for the Cambrian explosion, an abrupt period of adaptive radiation during which representatives of all modern animal phyla (except bryozoans) appeared. This occurred approximately 530 million years ago and lasted for about 10 million years.
The start of the Cambrian period has traditionally been defined as the emergence of shield-shaped arthropods called trilobites and early sponges (and the first reef-builders) called archeocyathids (literally "ancient cup"). Since then, its start has been redefined as an extinction event that marks the Cambrian-Precambrian boundary. The time of this event has been very precisely defined using changes in carbon isotope ratios occurring during this period, which indicate a large drop in planetary biomass.
Cambrian faunas were the first truly diverse faunas to evolve. The start of the Cambrian period is marked by the aforestated trilobites and archeocyathids, in addition to the slightly older small shelly fauna, the first known organisms with a hard shell, thought to be simple brachiopods. Early on in the Cambrian, distinctive trace fossils appear, including vertical burrows and other traces thought to be left by the first animals to develop hydrostatic skeletons. A hydrostatic skeleton is not a hard skeleton at all, but rather a fluid-filled body cavity which is attached to and is pulled on by surrounding muscles.
Throughout the Cambrian period, a variety of invertebrate animals — and a few early vertebrates in the form of jawless fish — evolved. These include jellyfish, early crustaceans, sponges, Priapulida, the extremely common Marrella (a relative of trilobites), the tiny arachnomorph Yohoia, the soft-bodied trilobite Naraoia, and "weird wonders" including the bizarre five-eyed Opabinia, a vaguely shrimp-like arthropod with a clawed appendage extending from its mouth, the huge (by Cambrian standards) Anomalocaris, which was the first apex predator, the spiny velvet worm Hallucigenia, and the odd, ruler-like Odontogriphus.
About half of the Cambrian fauna went extinct during the Cambrian–Ordovician extinction events at the end of the period. The brachiopods, conodonts, and trilobites were strongly impacted. Trilobites were never as numerous or diverse again.